1. Field
This application generally relates to the field of wireless communication systems, and more particularly to signals and protocols that enhance data transmission efficiency in such systems.
2. Related Art
The subject matter set forth herein is applicable to wireless communication systems generally. However, it has been developed primarily in the context of cellular telecommunication systems that provide high-speed connectivity including data and voice transport on both point-to-point and point-to-multipoint bases. First-generation (analog) and second-generation (digital) cellular networks were used primarily for communicating voice traffic via mobile cellular telephones, and thus maintained a focus on access methods for the efficient transport of voice information. With the rising popularity of the Internet, a third-generation (3G) wideband multimedia cellular network continues to be developed to transport both voice and data at much higher speeds than were previously available using the first and second generation wireless networks.
A Third Generation Partnership Project 2 (3GPP2) has been established by industry groups for the purpose of defining specifications to transition current code-division multiple-access (CDMA) wireless networks to the third generation, which is commonly referred to as CDMA2000. One such specification may be referred to as “CDMA2000 1x Revision D” (which may also be referred to as “CDMA2000 1x Rev D,” “cdma2000 Release D,” “IS-2000-D”, or “IS-2000-Rel. D”). The CDMA2000 1x Rev D specification, available from the 3GPP2, is incorporated by reference herein in its entirety for its teachings on communications protocols used in 3G wireless communications systems. The 3GPP2 is primarily concerned with defining specifications for CDMA systems such as are implemented in North America. A document specifying a somewhat different CDMA system, such as is used more commonly in Europe, may be identified as 3GPP TSG-RAN Release-5, and is hereby incorporated by reference for its teachings on CDMA systems. Also incorporated by reference is so much of 3GPP TSG-RAN Release-6 as has been made public, particularly including documents submitted in conjunction therewith and identified as R1-031268, R1-040534, and R1-040758.
As is well known, when a mobile station (“MS,” also referred to as User Equipment or “UE”) travels between different geographic cells or sectors of base stations (“BSs,” also referred to as “Node Bs”), the connection to the UE must be handed over (or handed off) between the different BSs. To this end, the UE may maintain an “active set” or list of BSs, the active set including one or more BSs with which the UE is presently in communication. Soft handoffs (“SHOs”) are “make before break” changes in connection from a UE, whereby communication begins with a new BS before communication is terminated with old BSs.
To effect an SHO, it may be desirable for a plurality of BSs, or even all of the BSs in the active set of a UE, to be in concurrent data communication with the UE. However, while this may be desirable to ensure that a new connection is established before an old connection is terminated, this concurrent data communication can lead to excessive transmission power. This is particularly true for signals that convey information uniquely between an MS and one BS of its active set. While such unique signals increase with each additional active set member, their uniqueness precludes any diversity benefit that accrues to the signals that are common to a plurality of active set BSs. Because transmission power on one channel effectively appears as noise on other channels, such excessive transmission power impairs the detection of other signals. Thus, any excess transmission power generally translates to reduced system connection capacity. It is therefore desirable to reduce or minimize unnecessary transmission power.
The use of automatic retransmission protocols that operate within a “physical” communication layer, such as the Hybrid Automatic Retransmission reQuest (H-ARQ) protocols, may enhance the efficiency of wireless data transmissions.
It is well known that Hybrid Automatic Retransmission reQuest (H-ARQ) protocols may provide significant gains when operating packet data channels. However, those gains come at a cost of increased power requirements. When a transmitting station (TS) transmits a packet to a receiving station (RS), H-ARQ procedures generally require the RS to promptly transmit an acknowledgment signal back to the TS in order to indicate whether the packet transmission was successful (ACK) or not (NACK). Such acknowledgment signals are unique between an MS and a particular member of its active set of BSs. Consequently, the power used to transmit such acknowledgment signals reduces downlink (DL) capacity both by consuming available transmitter power capacity, and by appearing as noise that decreases the detectability of other signals. Such additional power requirements for a DL ACK channel with a UE (or MS) in SHO have been analyzed by various authors, leading to the conclusion that excessive power may be required to satisfy the desired detection error probability (1% that ACK is incorrectly interpreted as NACK, and 0.1% that NACK is incorrectly interpreted as ACK) for BSs (or Node-Bs) in an active set. Furthermore, when “legs” of the SHO the SHO legs are significantly imbalanced, the power required for successful acknowledgement by a weaker BS (or Node-B) may exceed the total power available at such a weaker BS (each “leg” is a connection between the MS and one BS, so that there are as many legs as there are BSs in the active set of the MS).
Several techniques may be used to reduce the DL ACK peak power requirements, such as: “transmission scheduling;” a use of “ON/OFF” signaling, taking advantage of the fact that NACK occurs more frequently than ACK in most HARQ systems; etc. However, each such technique has associated disadvantages, with most being inefficient when employed in a system where significant numbers of UEs operate in autonomous mode. Furthermore, none of these techniques mitigate the problem of additional power requirements when plural SHO connections have significant leg imbalances from an MS to different BSs.
The methods and apparatus described herein address the above-described problems, and alleviate other difficulties as well.